This invention relates to holograms, and more particularly to a moisture insensitive reflection holographic article made of a material whose image is normally degraded by moisture.
Dichromated gelatin (DCG) is the best material presently available for reflection hologram display, but exposure to moisture causes loss of its image. For protection, it is usually sealed between two layers of glass with light curing epoxy. This light curing epoxy is expensive. The process is messy, and it is difficult not to trap bubbles in the epoxy layer. But because of DCG's high quality, hundreds of thousands of dichromated gelatin holograms on glass are made in the U.S.A. each year. Obtaining an easier protection from moisture would be advantageous.
It would also be advantageous to put the hologram onto a plastic substrate. A flexible plastic substrate would allow wider application such as mail, wallet, and book use. Long strips of holograms on plastic could be belt processed for a lower production cost. Because of the moisture sensitivity problem, commercial attempts in 1980 to apply DCG to plastic substrates by Dichromate Inc. of Emeryville, Calif. were abandoned. Their experiments with dip coating the holograms in common urethane sealers failed.
Because of this moisture problem, several patents have been issued to leading researchers from major corporations attempting to make DCG, or DCG-like holographic photopolymers, insensitive to ambient moisture. All four patents are different from the polymer moisture barriers discussed in this patent application.
U.S. Pat. No. 3,660,091 describes a chemical method to crosslink the gelatin and make it less sensitive to water.
U.S. Pat. No. 4,318,970 describes a technique for depositing a moisture barrier glass layer onto the preholographic substrate by electron or plasma beam deposition in a vacuum.
U.S. Pat. No. 4,330,604 discloses a moisture barrier technique similar to the previous patent in which Si3Ni4 is applied to the pre-hologram substrate instead of glass.
U.S. Pat. No. 4,535,041 discloses a technique of applying zirconium acetate and a fatty acid to a holographic photopolymer to prevent moisture from being able to penetrate the photopolymer.
None of these patents provides an inexpensive means of making a DCG hologram sufficiently insensitive to moisture, as is evidenced by the failure of DCG companies to give up the limited glass and epoxy packaging.